The present invention relates to a rope, and particularly to a rope which may be used for hoisting heavy objects, such as by cranes, derricks and similar devices.
There are already in widespread use cables or ropes of different constructions and of various materials. The selection of a particular rope for a particular purpose will depend on the environment in which the rope is to be used and also on the load which is applied to the rope. So, for example, there are already known ropes which are used in cranes, derricks, elevators and conveying arrangements, such ropes being almost exclusively made of steel wires which are intertwisted in form of yarns and strands to form the rope. While it is true that such conventional ropes are capable of withstanding substantial tensile stresses so that relatively heavy objects can be lifted using such ropes, such ropes are also disadvantageous in some respects. So, for instance, the steel material of such ropes is sensitive to the influences of the ambient atmosphere and it may corrode, so that it is necessary to inspect and protect such ropes or cables at frequent intervals, which involves substantial expense in terms of labor and material.
On the other hand, there are already known ropes of synthetic plastic material which are also capable of withstanding substantial loads. However, such ropes or cables of synthetic plastic material are disadvantageous in that they stretch, when subjected to load, to a great degree, such as by 35 to 50% It will be appreciated that such ropes or cables are not suited for use in hoisting devices, such as elevators, cranes, derricks, or in conveying arrangements, especially inasmuch as it is rather difficult to handle the objects to be lifted or transported because of the elastic yieldability of the rope of synthetic plastic material.
Another type of high tensile strength rope or cable of synthetic plastic material is known from the published German Pat. No. 2,222,312 which discloses a rope which includes a multitude of mainly carrying filaments of synthetic plastic material, and inserts or jackets of fibrous materials. The inserts or jackets are made of highly stretchable, but unstretched or only partially stretched materials, and such inserts or jackets are provided in such an amount and are so arranged that, when the rope of synthetic plastic material is overloaded, then a gradual reduction of the potential energy of the filaments of synthetic plastic material occurs in the overloaded regions of the rope of synthetic plastic material. In this rope, the inserts or jackets are made of metal or metal alloys, or of graded sections of not fully stretched identical or different synthetic plastic material monofilaments. This elastic rope is to be used predominantly on ships, and the reason for providing the inserts or jackets is to prevent damage to the ships or injury to the personnel which may otherwise occur subsequent to the rupture of the ropes due to the quick movement of the ends of the rope. However, even such a rope has a high degree of elasticity and, therefore, is not suited for use in connection with cranes, derricks, elevators and conveying arrangements.
A relatively recent development is synthetic plastic material of high tensile strength, which is a high-strength aromatic polyamide which is marketed under the designation PRD-49 or, more recently, Kevlar 49. It has been already proposed to make support ropes of such a synthetic plastic material. However, inasmuch as experiments with this new material have established that the maximum loading capacity of the rope cannot be obtained when the filaments of the high tensile strength material of which it consists are intertwisted or convoluted, this novel rope includes essentially parallel filaments. The parallel filaments are surrounded by a synthetic plastic material jacket which protects those filaments and holds them together. However, this rope or cable has only low flexibility, and short life span when subjected to bending, so that this rope is not suited for use in connection with cranes, derricks and similar lifting and transporting devices.